Polycarbonate resins are generally produced using bisphenols as a monomer ingredient, and are being extensively utilized as so-called engineering plastics in the fields of electrical/electronic parts, automotive parts, medical parts, building materials, films, sheets, bottles, optical recording media, lenses, etc. so as to take advantage of the superiority thereof such as transparency, heat resistance, and mechanical strength.
However, the conventional polycarbonate resins deteriorate in hue, transparency, and mechanical strength when used over a long period in places where the resins are exposed to ultraviolet rays or visible light. There hence have been limitations on outdoor use thereof and on use thereof in the vicinity of illuminators.
Techniques in which a benzophenone-based ultraviolet absorber, benzotriazole-based ultraviolet absorber, or benzoxazine-based ultraviolet absorber is added to a polycarbonate resin in order to overcome such problems are widely known (see, for example, non-patent document 1).
However, addition of such an ultraviolet absorber poses the following problems although the addition brings about improvements in hue retention through ultraviolet irradiation, etc. Namely, there have been problems, for example, that the addition of the ultraviolet absorber deteriorates the hue, heat resistance, and transparency which are inherent in the resin and that the ultraviolet absorber volatilizes during molding to foul the mold.
The bisphenol compounds for use in producing conventional polycarbonate resins have a benzene ring structure and hence show high absorption of ultraviolet rays. This leads to a deterioration in the light resistance of the polycarbonate resins. Consequently, use of monomer units derived from an aliphatic dihydroxy compound or alicyclic dihydroxy compound which has no benzene ring structure in the molecular framework or from a cyclic dihydroxy compound having an ether bond in the molecule, such as isosorbide, is expected to theoretically improve light resistance. In particular, polycarbonate resins produced using, as a monomer, isosorbide obtained from biomass resources have excellent heat resistance and mechanical strength, and many investigations thereon hence have come to be made in recent years (see, for example, patent documents 1 to 6).
However, since the aliphatic dihydroxy compound or alicyclic dihydroxy compound and the cyclic dihydroxy compound having an ether bond in the molecule, such as isosorbide, have no phenolic hydroxyl group, it is difficult to polymerize these compounds by the interfacial process which is widely known as a process for polycarbonate resin production using bisphenol A as a starting material. Usually, polycarbonate resins are produced from those compounds by the process which is called a transesterification process or a melt process. In this process, the dihydroxy compound and a carbonic diester, e.g., diphenyl carbonate, are subjected to transesterification at a high temperature of 200° C. or above in the presence of a basic catalyst, and the by-product, e.g., phenol, is removed from the system to allow the polymerization to proceed, thereby obtaining a polycarbonate resin. However, the polycarbonate resins obtained using monomers having no phenolic hydroxyl group, such as those shown above, have poor thermal stability as compared with polycarbonate resins obtained using monomers having phenolic hydroxyl groups, e.g., bisphenol A, and hence have had the following problem. The polycarbonate resins take a color during the polymerization or molding in which the resins are exposed to high temperatures and, as a result, the polycarbonate resins come to absorb ultraviolet rays and visible light and hence have impaired light resistance. Especially when a monomer having an ether bond in the molecule, such as isosorbide, was used, the polycarbonate resin considerably deteriorates in hue. A significant improvement has been desired.
Meanwhile, as stated above, polycarbonate resins are extensively utilized as so-called engineering plastics in the fields of electrical/electronic parts and automotive parts and in optical fields such as optical recording media, lenses, etc. However, for use as optical compensation films for flat panel displays, which are rapidly spreading recently, the films have come to be required to have higher optical properties including low birefringence and a low photoelastic coefficient. The existing aromatic polycarbonates have come to be unable to meet the requirement.
Conventional polycarbonates are produced from starting materials derived from petroleum resources. In recent years, however, there is a fear about depletion of petroleum resources, and there is a need for a polycarbonate produced using a starting material obtained from biomass resources including plants. In addition, there is a fear that the global warming caused by increases in carbon dioxide emission and by carbon dioxide accumulation may bring about climate changes and the like. Also from this standpoint, there is a desire for development of a polycarbonate which is produced from a plant-derived monomer and which is carbon-neutral even when discarded after use.
Under these circumstances, a process has been proposed in which a special dihydroxy compound is subjected as a monomer ingredient to transesterification with a carbonic diester to obtain a polycarbonate while removing the by-product monohydroxy compound by distillation under vacuum, as described in patent documents 1 to 6.
However, such a dihydroxy compound having a special structure has a lower boiling point than bisphenols and hence volatilizes considerably during the transesterification reaction, which is conducted at a high temperature and a reduced pressure. The volatilization thereof results not only in a decrease in material unit but also in a problem that it is difficult to regulate the concentration of end groups, which affects quality, to a given value. Furthermore, there has been a problem that when a plurality of dihydroxy compounds are used, the molar proportions of the dihydroxy compounds used change during the polymerization, making it impossible to obtain a polycarbonate resin having a desired molecular weight and a desired composition.
Expedients which may be usable for overcoming those problems include to lower the polymerization temperature and to lessen the degree of vacuum. However, use of these expedients poses a dilemma that monomer volatilization is inhibited but a decrease in productivity results.
A technique in which a polymerization reactor having a specific reflux condenser is used has also been proposed (see, for example, patent document 7). However, the improvement in material unit is still on an unsatisfactory level, and a further improvement is desired.
In addition, the by-product monohydroxy compound, when removed by distillation, deprives the system of a large amount of latent heat of vaporization. Consequently, for maintaining a given polymerization temperature, it is necessary to heat the system with a heating medium (heat medium). In a larger-scale apparatus, however, the heat-transfer surface within the reactor has a reduced area per unit amount of the liquid reaction mixture and, hence, it becomes necessary to heat the reactor with a heat medium having a higher temperature. This means that the part of the liquid reaction mixture which is in contact with the surface of the wall through which the heat medium flows is heated at a higher temperature. Namely, not only volatilization of the low-boiling dihydroxy compound which is in contact with the wall surface is considerably accelerated, but also there is a problem that the heating causes thermal deterioration in the vicinity of the wall surface, resulting in a quality deterioration. This problem becomes more serious as the scale increases.